A CFD Investigation of Emissions Formation in HCCI Engines, Including Detailed NOx Chemistry

Three-dimensional time-dependent CFD simulations of autoignition and emissions are reported for an idealized engine configuration under HCCI-like operating conditions. The emphasis is on NOx emissions. Detailed NOx chemistry is integrated with skeletal autoignition mechanisms for n-heptane and iso-octane fuels. A storage/retrieval scheme is used to accelerate the computation of chemical source terms, and turbulence/chemistry interactions are treated using a transported probability density function (PDF) method. Simulations include direct in-cylinder fuel injection, and feature direct coupling between the stochastic Lagrangian fuel-spray model and the gas-phase stochastic Lagrangian PDF method. For the conditions simulated, consideration of turbulence/chemistry interactions is essential. Simulations that ignore these interactions fail to capture global heat release and ignition timing, in addition to emissions. For these lean, low-temperature operating conditions, engine-out NOx levels are low and NOx pathways other than thermal NO are dominant. Engine-out NO2 levels exceed engine-out NO levels in some cases. In-cylinder inhomogeneity and unmixedness must be considered for accurate emissions predictions. These findings are consistent with results that have been reported recently in the HCCI engine literature.